The present invention relates to a power tool, in particular a hand-operated chiseling power tool.
In the case of hand-held chiseling power tools, chiseling action is supposed to be suspended when a chisel is lifted off a workpiece. In the case of striking mechanisms that operate pneumatically, a pneumatic spring can be deactivated by means of additional ventilation openings, which are only opened if the chisel is disengaged. A striker, also called an intermediate striking device or anvil, is supposed to remain away from the ventilation openings for this purpose after an empty impact. However, this is not the case to some extent due to the rebound of the striker on a forward limit stop.
A power tool according to the invention has a striker, which is guided along an axis in a guide. A pneumatic chamber has a volume which varies with a movement of the striker along the axis. A pneumatic chamber is closed by the striker, the guide and a valve device actuated by its own medium. The volume of the pneumatic chamber varies with a movement of the striker along the axis. The valve device actuated by its own medium has, in a flow channel between the striker and the guide, a sealing element that is moveable between two positions in a bearing along the axis. The flow channel has a first cross-sectional area in a first of the two positions of the sealing element adjacent to a first mating surface of the bearing, and the flow channel has a second cross-sectional area in a second of the two positions of the sealing element adjacent to a second mating surface of the bearing offset from the first mating surface along the axis. The second cross-sectional area is greater than the first cross-sectional area. The valve device actuated by its own medium may have, for example, a groove embedded in the striker or in the guide, and a sealing element. The sealing element is moveable in the groove along the axis between a first and a second groove wall. The flow channel of the valve device has the first cross-sectional area in a first position of the sealing element adjacent to the first groove wall and the second cross-sectional area in a second position of the sealing element adjacent to the second groove wall, which is greater than the first cross-sectional area. Adjacent to the first groove wall, the sealing element closes or throttles an air flow into or out of the pneumatic chamber. The striker experiences a braking effect because of the closed pneumatic chamber when it slides back into the tool receptacle. Adjacent to the second groove wall, a greater air flow through the second cross-sectional area of the flow channel is possible. In the case of a movement in the impact direction, the valve device makes a pressure equalization possible in the pneumatic chamber, which is why no braking effect occurs.
One embodiment provides that a volume of the pneumatic chamber is increasing in the case of a movement of the striker in the impact direction and the first mating surface of the bearing is facing the pneumatic chamber, e.g., the groove with the second groove wall is arranged facing the pneumatic chamber. In the case of an air flow out of the pneumatic chamber, the sealing element is pushed in the direction of the mating surface of the bearing facing the pneumatic chamber. With this first variant, air is able to flow into the pneumatic chamber, when the striker moves forward and the volume increases. When the volume of the pneumatic chamber is decreasing in the case of a movement of the striker in the impact direction, the second mating surface of the bearing is facing the pneumatic chamber, e.g., the groove with the first groove wall is arranged facing the pneumatic chamber. A further embodiment provides for two pneumatic chambers, which are connected by the valve device actuated by its own medium.
One embodiment provides that the flow channel runs between the first mating surface of the bearing and a first mating surface of the sealing element assigned to the first mating surface of the bearing and between the second mating surface of the bearing and a second mating surface of the sealing element assigned to the second mating surface of the bearing. The first cross-sectional area of the flow channel is determined by the space between the first mating surfaces of the bearing and the sealing element, when these are adjacent to each other. The second mating surface of the bearing and/or a mating surface, that is the second mating surface, of the sealing element assigned to the second mating surface of the bearing may have narrow channels running at least in part radially, i.e., perpendicularly, to the axis. The narrow channels define a second cross-sectional area that is greater than zero and make an air exchange possible into or out of the pneumatic chamber, even if the sealing element is adjacent to the second groove wall. The two second mating surfaces of the bearing and of the sealing element close flush only in part, e.g., due to the narrow channels. The second cross-sectional area is not equal to zero and an airflow may flow through the flow channel. If the two first mating surfaces are flush with each other, the first cross-sectional area is equal to zero. The groove and the sealing element may run annularly around the axis and, in the first position, the sealing element touches the guide and the striker respectively along a closed line around the axis.
One embodiment provides that a channel runs from the first groove wall to the second groove wall between a groove base of the groove and the sealing element. The flow channel of the valve runs between the sealing element and the body in which the groove is introduced.
In one embodiment, the first groove wall is inclined with respect to the axis by less than 60 degrees and the second groove wall is inclined with respect to the axis by at least 80 degrees.
One embodiment provides that the first cross-sectional area of the flow channel is a maximum of one tenth of the second cross-sectional area of the flow channel.
One embodiment provides that the striker has a prismatic first section and a second section with a larger cross-sectional area as compared to the first section, wherein the valve device is arranged in the second section of the striker. Bodies having a cross-section that is constant along an axis, e.g., cylinders, are prismatic.
One embodiment provides that a seal between the striker and the guide and that is offset from the valve device actuated by its own medium along the axis for sealing the pneumatic chamber is provided, wherein the valve device actuated by its own medium and the seal are arranged at different distances from the axis.
One embodiment has a throttle, which connects the pneumatic chamber with an air reservoir. An effective cross-sectional area of the pneumatic chamber, defined by the differential of the volume of the pneumatic chamber in the impact direction is greater than one hundred times a cross-sectional area of the throttle. The striker is moved parallel to the axis, whereby a volume change of the pneumatic chamber is produced proportional to the displacement along the axis and the effective cross-sectional area. The effective cross-sectional area can be determined by the mathematical operation of differentiation in the movement or impact direction. In the case of a cylindrical guide and a cylindrical striker, the effective cross-sectional area corresponds to the largest cross-sectional area perpendicular to the axis. The ratio of the effective cross-sectional area of the pneumatic chamber to the cross-sectional area of the throttle determines a relative flow speed of the air in the throttle related to the speed of the striker. Starting at this relative flow speed, the air can escape quickly enough from the pneumatic chamber without a drop in pressure developing with respect to the environment. It was recognized that an absolute speed of the air in the throttle cannot be exceeded. However, the throttle appears to block a limit value of the absolute speed. The ratio of a hundred times, preferably three-hundred times, is selected so that, in the case of a striker driven by the striking mechanism, the absolute speed of the air in the throttle is reached; in the case of a striker moved manually, the absolute speed is fallen short of considerably. As a result, the throttle blocks when the striker strikes, and opens when the striker is moved manually.
In one embodiment, the valve device may be configured as a throttle valve device. An effective cross-sectional area of the pneumatic chamber defined by the differential of the volume of the pneumatic chamber in the impact direction is greater than a hundred times of a cross-sectional area of the flow channel. The first mating surface of the bearing and/or a mating surface of the sealing element assigned to the first mating surface of the bearing may have narrow channels running radially perpendicularly to the axis at least in part. A total of their cross-sectional area is less than one hundredth of the effective cross-sectional area of the pneumatic chamber.
One embodiment has a pneumatic striking mechanism, which is arranged percussively with its impacting piston in the impact direction on the striker. The striker is an impact body or an anvil moveable along the axis, which is arranged between a striking device of a pneumatic striking mechanism and a tool inserted into a tool receptacle.
The following description explains the invention on the basis of exemplary embodiments and figures.